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DuctSox Fabric Ducting

Staff | September 28, 2010

When thinking of building materials, we think of wood, steel and concrete. Fabric is for the interior decorator. But according to Randall E. Fromberg, president of Fromberg Associates Ltd., fabric duct is the fastest growing air distribution alternative in open-ceiling architecture designs, which include warehouses, industrial plants, big box retail, sports arenas, gymnasiums, and large lobbies.

While many architects and consulting engineers still don't specify fabric ductwork, the architect firm Fromberg Associates Ltd. of Austin has specified fabric duct in over 10 gymnasiums and cafeterias it has designed over the last eight years.

"I guess we're a little more cutting edge than some other architects; we are always looking for ways to add value to our client's budget," Fromberg admitted.

Fromberg recommended fabric HVAC ductwork for the Florence's new $2.4-million high school gymnasium. Florence is a rural community between Georgetown and Killeen with a school enrollment of only 1,058. They had built a new high school four years ago, but had to continue using the old gymnasium until residents saw the need for better facilities, according to school superintendent, John Van Dever. Voters passed a $4.5-million school bond in 2005. In addition to the new gymnasium, the district is also building a new maintenance facility, bus garage, a new wing on the high school, and additional classrooms for the elementary school. In order to make the district's greenbacks go farther, the architect and design team suggested economical materials.

About the fabric ductwork, Van Dever said, "We were skeptical at first." Fromberg and representatives of the construction management firm, Kencon Constructors Inc. of San Antonio, convinced the Florence ISD skeptics of fabric ductwork advantages by taking an entourage of board members and administrators to see the system in place in another school district.

Fabric duct can easily be a green product in more ways than one. Fabric saves using precious earth resources. Although recycling metals is good, finding a plentiful alternative material is better, especially a more economical material. When the time comes for remodeling, retrofitting or razing, fabric ducts take less space in landfills.

Fabric duct is also economical in cost and in shipping. The woven polyester tubing adequate for an entire gymnasium can be shipped in only a couple of boxes, according to Jeffrey L. Klopfenstein, PE, LEED® AP Manager of Engineering for DuctSox Corporation.

"That amounts to only a fraction of the volume and fossil fuel needed to transport metal ductwork. We have a lot to offer the green initiative," Klopfenstein added.

The Florence project's consulting engineer, Gil Kent P.E., president of mechanical/electrical/plumbing engineering firm Kent Consulting Engineers LP, Austin, was no stranger to fabric duct either after specifying it on over two dozen projects. While Fromberg likes the streamlined aesthetics, Kent specifies fabric duct because of cost-saving benefits as well as performance characteristics. Tests show that the regular hole vents in fabric ducting distribute air more evenly than metal vents. Thus it fit Kent's energy-efficient HVAC system design that uses energy recovery to treat outside air and save Florence over 50 percent in air conditioning operating costs versus conventional DX equipment designs.

Kent specified DuctSox Corp., Dubuque, Iowa, commercial-grade Verona fabric duct with Comfort Flow air dispersion that allows 85 percent of the air to flow evenly through linear vents spanning the entire length of duct. The remaining 15 percent flows through the fabric to eliminate condensation and surface dust accumulations — always a problem for Texas gyms and natatoriums.

Another benefit of fabric ducts in regard to dust is its launderability, according to Klopfenstein. "Every three years or so take it down, turn it inside out, and launder it in a commercial washer to clean off the accumulated dust and extend its life expectancy." According to Klopfenstein, warranties on the varying grades of DuctSox ranges from one year to 20 years.

The air distribution configuration consists of two 165-foot-long runs of 36-inch-diameter duct with one L-Vent linear diffuser along the entire length. Unlike metal duct registers every 10 feet, the linear diffusion produces an even air distribution without drafts that efficiently results in less HVAC equipment run-time.

While the continuing escalation in world market metal costs have made fabric ductwork cheaper than double-wall spiral metal duct, Florence saved the bulk of its costs on labor, which according to mechanical contractor, Lockridge-Priest Inc. of Temple, was approximately 50-percent less than installing metal duct. Installing fabric was also easier than metal even though Kent specified a duct configuration with many radiuses to emulate the slope of the gym's gable roof and keep the ductwork more remote from the basketball/volleyball playing areas.

"It only took two days to install because the learning curve was short and once the suspension system is installed, the actual fabric duct installation goes fast," said Jim Kruse, vice president of Lockridge-Priest Inc., which had no prior experience with fabric duct, but plans to use it in the future on future design/build projects.

The ductwork also sports custom school colors of blue and white as well as silk-screened logos of the school's name, the "Buffaloes."

"When the ventilation is idle it looks like a banner you would typically see in a gym," added Fromberg.

Instead of running large make-up air conditioning equipment and continually conditioning required outside air, Kent specified a Greenheck, Scofield, Wis., energy recovery direct expansion (DX) cooling/gas-fired heating (ERCH) that removes Texas' notorious humidity from outside air with mechanical and enthalpy dehumidification. The two 29-ton (SEER-25) ERCH units are activated by CO2 sensors and efficiently throttled by on-board microprocessor-controlled variable speed drives that distribute air through wall grills and only operate during occupancy periods.

Consequently the ERCHs greatly reduce the cooling load of the building's four 12.5-ton Trane, Tyler, Texas, DX units that each provide 5,000-cfm to the DuctSox fabric duct system. Since the ERCH units use DX coils to condense moisture out of the outside air, the resulting "free" 55-degrees Fahrenheit discharge air many times accounts for the majority of the space's cooling. "These units offer a payback of less than a year and cut operating costs by 50 percent versus more outside-air conventional systems that run at the same schedule and don't use energy recovery," said Kent.

Typically Kent prefers to control this type of HVAC design configuration with a Direct Digital Control (DDC) system; however, controlling and monitoring capabilities were value-engineered with a seven-day time clock by Honeywell, Minneapolis, Minn.

Another cost-saving feature is Kent's specification of placing Greenheck ERCH and Trane air-handling units indoors on platforms, which provides more floor space for storage and places the units closer to the gym's duct runs to reduce labor and materials.

The Florence ISD gym addition is a good example of an architect, consulting engineer and contractor teaming up to save a school district significant construction costs without sacrificing critical IAQ and aesthetics in the process.